1. Field of the Invention
The present invention relates to a dynamic loading system for piles which serve as a foundation of a structure, a dynamic loading method for estimating the bearing capacity of a pile, and a dynamic loading test method.
2. Description of the Background Art
Soil under strain can be treated as an elastic body when the strain is equal to or smaller than 10xe2x88x924, or in a region of 10xe2x88x925 if the soil is relatively soft. When the soil is subjected to a strain exceeding these values, its plastic nature gains greater importance.
When a load exerted on a pile driven into the ground is small and strain occurring in the pile is remarkably small, strain occurring in the ground which is in contact with the pile is also remarkably small, so that the ground can be treated as an elastic body. In this case, the strain in both the pile and the ground is eliminated and they resume their original form when the load is removed. The load applied in this case falls within a range not exceeding their ultimate bearing capacity.
When the applied load is increased, the strain occurring in the pile increases, also causing a large amount of strain in the ground. When the plastic nature of the ground becomes of greater importance as a consequence, plastic deformation occurs in the ground which is in contact with the pile. The plastic deformation of the ground does not disappear and the pile does not return to its original position even when the load is removed. The load applied in this case falls within a range exceeding the ultimate bearing capacity.
Conventionally, stationary loading tests, dynamic loading tests and rapid loading tests are performed as methods for evaluating the ultimate bearing capacity (hereinafter referred to as the bearing capacity) of a pile.
The stationary loading test is a method of determining the stationary bearing capacity of a pile from the relationship between a load and the amount of sinking of the pile when the load is exerted on the pile to be tested.
FIG. 15 is a diagram showing the structure of a conventional stationary loading system 1000 for measuring the bearing capacity of a pile. In this Figure, designated by the numeral 1001 is a test pile whose bearing capacity is to be measured, designated by the numeral 1002 is one of reacting piles, designated by the numeral 1003 is a loading beam, designated by the numeral 1004 is a hydraulic jack, designated by the numeral 1005 is a control unit for controlling the hydraulic jack 1004, and designated by the numeral 1006 is a gauge. Further, the marking GL indicates the ground level.
The stationary loading test method carried out by the stationary loading system 1000 thus constructed is described in the following. As shown in the Figure, the reacting piles 1002 are provided around the test pile 1001 to be tested. While supporting a loaded weight with the reacting piles 1002, a load is applied to the test pile 1001. This load is applied by the hydraulic jack 1004 which is provided between the loading beam 1003 supported by the reacting piles 1002 and the test pile 1001. The hydraulic jack 1004 applies the load to the test pile 1001 in a vertical direction according to a control quantity fed from the control unit 1005. After loading, the amount of sinking of the test pile 1001 is measured by the gage 1006 and the bearing capacity is assessed from the relationship between the amount of the loaded weight and the amount of sinking.
Although the bearing capacity of a test pile can be measured with high reliability by this kind of conventional stationary loading test method, it necessitates considerably large-scale work, such as driving the reacting piles and installing the loading beam for producing a sufficient load to be applied to the test pile, involving the provision of a sizable testing facility. In addition, movement of the facility requires considerable expenses and time, resulting in extremely poor efficiency. It has therefore been difficult in practice to measure the bearing capacities of a large number of piles.
In a conventional dynamic loading test method, on the other hand, a load is dynamically exerted on a test pile by hammering its head and the bearing capacity of the test pile is estimated by analyzing a response obtained by a vibration sensor mounted on the pile head.
Although this kind of dynamic loading test method does not require a large-scale facility like that of the stationary loading test method, loading time is as short as a few milliseconds and the wavelength of elastic vibrations produced is sufficiently short compared to the length of the test pile. Therefore, it is necessary to carry out a complicated analytical treatment based on a wave theory by regarding the pile body as a one-dimensional elastic body in a stage of estimating the bearing capacity from waveforms detected by the vibration sensor. In addition, estimated values of the bearing capacity fluctuate to a large extent because information obtained from the pile head is limited.
In a conventional rapid loading test method, a load is exerted on a test pile by exploding a propellant like an explosive and applying a resultant impact force to the pile head. In this method, it is possible to obtain about ten times as longer a loading time as in the conventional dynamic loading test method and apply the load in a more stationary state. This method has problems in practical applications, however, because it involves a lot of limitations including the need for careful handling of the explosive.
Another conventional dynamic loading test method disclosed in Japanese Laid-open Patent Publication No. 10-153504 is described in the following with reference to FIG. 16 as an example of a method intended to overcome the problems of the aforementioned dynamic loading test method.
In hammering a pile head by dynamic loading of this test method, a load is exerted at a desired frequency by successively dropping a plurality of split hammer blocks at regular intervals.
In a stationary loading system shown in FIG. 16, a guide shaft 2002 is installed upright on an anvil 2001 and a hook 2003 is provided at the top of the guide shaft 2002. The anvil 2001 has at its lower portion a pile cap 2004 which is fitted over the head of a pile P. Measuring equipment, such as a load meter 2005, is provided between the anvil 2001 and the pile head for measuring the load and a displacement meter 2006 is provided on a side surface of the pile cap 2004 for measuring the displacement of the pile head. A hammer includes a plurality of hammer blocks M1-Mn, each hammer block M having a through hole 2007 at a central position for passing the guide shaft 2002.
Next, operation of this stationary loading system is described below.
The hammer blocks M mounted on the guide shaft 2002 are hung by wire ropes 2008 which are hooked on the hook 2003. Each wire rope 2008 is equipped with an unillustrated latch and the hammer blocks M are retained at regular intervals d. The hammer blocks M are simultaneously released by disengaging the hook 2003. As a result, the individual hammer blocks M fall successively onto the pile head striking against it and exerting a series of loads thereupon. The loads are measured by the load meter 2005 and the displacement of the pile head is determined by the displacement meter 2006.
In the aforementioned loading method, the regular spacing d between the successive hammer blocks M defines uniform time intervals between them, so that dropping time intervals can be varied by altering the spacing d. Thus, this method makes it possible to control the frequency of the entire loads and apply the loads in a state much closer to stationary conditions.
Even by the aforementioned improved dynamic loading test method the prior art, however, it is difficult to continually apply loads for an extended period of time. In addition, it is necessary to adjust the spacing between hammer blocks for controlling the frequency of the loads and to adjust the mass of the hammer blocks for controlling impact forces produced when the successive hammer blocks strike against the pile head, relusting in complicated work and poor efficiency.
This invention is intended to provide means for overcoming the aforementioned problems of the prior art. Specifically, it is an object of the invention to provide a dynamic loading method and a dynamic loading test method which make it possible to conduct a loading test of a pile with good controllability and ease at low cost and to estimate the bearing capacity of the pile with high reliability without the need for complicated analytical treatment. It is another object of the invention to provide a structure of a dynamic loading system which enables such dynamic loading.
According to the invention, a dynamic loading system for a pile includes a magnetostrictive vibrator formed of a magnetostrictive element which becomes strained when placed in a magnetic field and an exciting coil for producing the magnetic field in the magnetostrictive element. Further including a joint mechanism for connecting the magnetostrictive vibrator to the head of the pile, a power supply unit for feeding an electric current into the magnetostrictive vibrator and a control unit for controlling the frequency and amplitude of the electric current, the dynamic loading system vibrates the pile by a strain occurring in the magnetostrictive vibrator.
This dynamic loading system of the invention makes it possible to control vibrations produced in the pile in a desired fashion with a simple and low-cost system configuration. It also makes it possible to efficiently perform dynamic loading and dynamic loading tests with high reliability.
According to the invention, a dynamic loading method for a pile includes feeding an electric current into an exciting coil of a magnetostrictive vibrator which is connected to the head of the pile, and transmitting a strain occurring in the magnetostrictive vibrator due to a magnetic field to the pile in the form of vibrations to thereby vibrate the pile.
This method makes it possible to perform dynamic loading and a dynamic loading test with high reliability, in which vibrations produced in the pile can be controlled in a desired fashion.
According to the invention, a dynamic loading test method for a pile includes vibrating the pile by the aforementioned dynamic loading method, detecting vibrations produced in the ground around the pile, and estimating the bearing capacity of the pile.
This method makes it possible to conduct a loading test of the pile with good controllability and ease at low cost and estimate the bearing capacity of the pile with high reliability without the need for complicated analytical treatment.
In another dynamic loading test method for a pile, the pile is vibrated by feeding an electric current whose amplitude varies with time into the exciting coil of the magnetostrictive vibrator using the aforementioned dynamic loading method. Then, vibrations produced in the pile itself and in the ground around the pile are detected by respective vibration sensors, and the bearing capacity of the pile is estimated by calculating a transfer function from sensing signals of the respective vibration sensors.
This method makes it possible to conduct a loading test of the pile with good controllability and ease at low cost and estimate the bearing capacity of the pile with high reliability and certainty by estimating individual parameters of a theoretical model applied to a contact surface between a peripheral surface of the pile and the ground without the need for complicated analytical treatment.
Still another dynamic loading test method for a pile includes first to fifth steps which are described in the following. The first step determines the bearing capacity of a reference pile driven into the ground in the vicinity of the pile by a stationary loading test method. In the second step, the reference pile is vibrated by the aforementioned dynamic loading method and vibrations produced in the ground around the reference pile are detected. In the third step, the bearing capacity of the reference pile obtained in the first step and a vibration sensing signal obtained in the second step are memorized together with information on their mutual relationship. In the fourth step, the pile is vibrated in the same way as the second step and vibrations produced in the ground are detected. In the fifth step, the bearing capacity of the pile is estimated based on a vibration sensing signal obtained in the fourth step with reference to information memorized in the third step.
This method makes it possible to conduct a loading test of the pile with good controllability and ease at low cost and estimate the bearing capacity of the pile with ease and high reliability without being adversely affected by stratum formation or the ground.
These and other objects, features and advantages of the invention will be more apparently understood from the following detailed description if read in conjunction with the accompanying drawings.